RHEL-15110: Only one domain socket listener should be allowed

* It is another bug fix. It was possible to start two domain
  socket listeners using D-Bus method Start(). This should not
  be allowed. There should be only one client application
  trying to do registration.

src/rhsmlib/dbus/objects/register.py

@@ -104,6 +104,12 @@ def __init__(self, conn=None, object_path=None, bus_name=None):
     def Start(self, locale, sender=None):
         locale = dbus_utils.dbus_to_python(locale, expected_type=str)
         Locale.set(locale)
+
+        # When somebody else started domain socket listener, then
+        if self.impl.server is not None:
+            log.warning(f"domain socket listener already running, opened by: {self.impl.server.sender}")
+            raise exceptions.Failed(f"domain socket listener already running")
+
         self._caller_uid = self._bus_interface.GetConnectionUnixUser(sender)
 
         address: str = self.impl.start(sender)